Concrete slabs are used by various structures, including highways, airport runways, parking lots, building floors, sidewalks, and driveways. The methods used to construct these various different structures typically all require that the wet concrete mixture be poured, leveled and compacted.
The concrete may be leveled and compacted by screeding. The screeding process may be accomplished by the use of forms, most commonly 2 by 6 or 2 by 8 pieces of wood that are positioned in a parallel manner at the desired width. This form then operates to contain the poured concrete in a lateral area that is to be covered by the concrete slab. When the required amount of concrete is thus positioned, it is then necessary to level it off to the height of the forms. It is this later process in which the screed is employed. In this method, the leveling process is accomplished by moving a flat piece of material spanning the two parallel forms in a back and forth manner. This operation serves to move any of the excess concrete that extends above the upper surfaces of the forms either into any low areas or off of the prospective slab altogether.
While the manual method described above works well enough on small jobs such as the repair of short sections of sidewalk, it has numerous deficiencies. The first of these is, that even in small jobs, it is labor intensive and therefore costly over the long term. Additionally, the use of a manual screed is not very effective at distributing and compacting the concrete within the form, therefore producing a finished slab of a lesser quality than is generally desired.
Powered screeds may be employed to assist in the screeding process. One type of powered screed is a powered roller screed. The powered roller screed generally consists of a screed roller (e.g., an elongated tube) that is rotationally driven by an attached motor. In operation, the screed roller is positioned over the raw concrete with each end of the screed roller positioned on the upper edges of the forms. The screed roller is then moved along the top of the forms in a direction that is opposite the rotational motion of the screed roller at its point of contact with the concrete. This apparatus produces a smooth and flat finish to the concrete. This method is generally limited to producing concrete slabs in sections that are not wider than the width of the screed roller, since each end of the screed roller must ride on a form.
Screeding may also be accomplished without the use of the aforementioned forms. Such a process is known as wet or free screeding. In wet screeding, generally a free-floating elongate blade is moved over the freshly poured concrete to compact and level the concrete. The wet screed apparatus may include a vibration producing mechanism to vibrate the blade, which may aid in compacting the concrete. Pads, posts or other indicators may be used to help the wet screed operator level the concrete at a particular height. Since no forms are needed, wet screeding is not limited to producing sections that are thinner than the length of the blade. Accordingly, wet screeding may be used to screed large concrete pads, where the use of powered roller screeds may be impractical. However, wet screeding generally does not produce as high a quality surface as is generally achieved using a screed roller.